Macro delegate_value_encoding 

macro_rules! delegate_value_encoding {
    (
        delegate from ($from_ty:ty) to ($to_ty:ty) for type ($value_ty:ty)
        $(with where clause ($($where_clause:tt)+))?
        $(with generics ($($value_generics:tt)*))?
    ) => { ... };
    (
        delegate from ($from_ty:ty) to ($to_ty:ty) for type ($value_ty:ty) including distinguished
        $(with where clause for relaxed ($($relaxed_where:tt)+))?
        $(with where clause for distinguished ($($distinguished_where:tt)+))?
        $(with generics ($($value_generics:tt)*))?
    ) => { ... };
    (
        encoding ($encoding:ty) borrows type ($ty:ty) as owned
        $(with where clause ($($where_clause:tt)+))?
        $(with generics ($($impl_generics:tt)*))?
    ) => { ... };
    (
        encoding ($encoding:ty) borrows type ($ty:ty) as owned including distinguished
        $(with where clause ($($where_clause:tt)+))?
        $(with generics ($($impl_generics:tt)*))?
    ) => { ... };
}

Expressly delegates support for encoding values from one encoding to another, but not message fields themselves. Most built-in encodings in bilrost have the ability to encode any type they can encode as a value as a message field as well; a notable exception is the special Proxied encoding, which cannot.

Also includes support for providing borrowed value decoding implementations that delegate to the owned implementation, for types which have no borrowed representation. Most types supported by bilrost cannot be meaningfully borrowed and delegate their borrowed decoding impls back to the owned impls this way.

Delegation is implemented by macro type-by-type rather than as a blanket impl (such as borrowed encoding whenever owned decoding exists) right now because we want impl RawMessage to provide value encoding and decoding in the General encodings. This continues to preclude any other blanket trait delegation.